1. Field of the Inventions
The present application relates to a method of steering a watercraft propulsion device using an electric motor.
2. Description of Related Art
Conventionally, cable and hydraulic manual steering systems are used for steering watercraft propulsion devices such as outboard motors and stern drives (hereinafter “outboard motors”). The cable-type steering systems can generate high operational loads. Thus, the hydraulic manual steering systems are more commonly used.
In hydraulic manual steering systems, it is not practicable to include control systems for optimizing steering angles in accordance with watercraft speed. In addition, since hydraulic piping is required for such systems, additional space for the piping is required in the hull. Thus, the design of the system structure is complicated and construction and servicing are time-consuming.
More recently, a “Drive-By-Wire” (DBW) type system has been developed in which steering is electronically controlled using a steering drive unit including an electric motor (see Japanese Patent Publication No. Hei 4-38297, for example). In this system, an outboard motor is mounted to a transom plate and includes a steering drive unit having an electric motor which drives the outboard motor to rotate about a swivel shaft. The method of operating the system includes calculating a control quantity for the steering drive unit in accordance with the degree of operator's steering displacement; and operating the steering drive unit based on the calculated control quantity.
In such conventional method of steering an outboard motor, a control quantity can be directly and unequivocally correlated to the steering wheel displacement. The control command signal, based on the steering angle as the control quantity, is sent to the steering drive unit to control the electric motor so as to maintain the steering drive unit in the desired orientation.
When the electric motor is controlled in a manner such that the control amount for driving the electric motor is directly correlated to a steering angle of the propulsion device in accordance with the steering wheel displacement, the operator might attempt numerous course corrections requiring a large amount of power to move the outboard motor. For example, in some operating conditions, the direction of the watercraft can frequently change under the influence of waves and/or wind on the watercraft. These changes can cause the operator to move the steering wheel frequently in an attempt to stay on the desired course. However, depending on the experience of the operator, this can result in more steering movements than necessary, thus wasting electrical power.
In addition, when the control physical quantity is constant, no drive-control of the electric motor is allowed in accordance with a steering angle using a control physical quantity, e.g. steering torque, angular speed, a tactical diameter, or orientation, as a target control amount, which provides an optimum steering feeling in accordance with operating states of the watercraft or an ambience when the watercraft enters or leaves a port, cruises at sea, or the like.